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We know that friction is of two types - static and kinetic. Static friction acts when there is no relative motion between the surfaces in contact. Kinetic friction takes place when surfaces rub against each other. I was wondering whether the work done by the kinetic friction can be positive, negative or zero.

  • Positive work - When an object is placed on a rapidly moving belt, it moves along with the belt but with slipping (relative motion between the surfaces exist) when there is no enough friction to prevent slipping. Here the work done by the kinetic friction is positive, as the direction of frictional force and the displacement is same.

  • Negative work - Work done by kinetic friction, when an object moving on a rough surface slows down, is negative as the direction of friction and displacement are opposite to each other.

I'm unable to think of any circumstances when the work done by kinetic friction is zero because of the following reasons:

  • Work done on an object is zero if displacement is zero. In our case, if displacement is zero, the frictional force acting on the object is static and not kinetic in nature.

  • Work done is also zero when the force and displacement are perpendicular to each other. The only example I am aware of is circular motion. As the point at which the wheel touches the ground is at rest. The nature of friction is again static.

So, can the work by kinetic friction on an object be zero?

Please note: I read the answers for the following two related questions. There is no clear explanation on the two aspects of friction (static and kinetic) in those answers. Simply they don't have enough details.

Vishnu
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5 Answers5

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Hold a piece of wood against a sanding belt. In your frame, the block is not moving, but

  • kinetic friction is exerting a force: you have to hold the block still

  • energy is transferred: the block gets hot, and pieces are pulled off it

Bob Jacobsen
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Yes work done by kinetic friction may be zero for example:- consider a block slipping on ground work done by kinetic friction will be negative in ground frame but now observe the block w.r.t block itself now work done by each and every force will be zero as displacement of block w.r.t itself is zero.

Shreyansh Pathak
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Work done by all dissipative forces are always negative, including kinetic friction, because they always reduces kinetic energy. When you place object on moving belt - it starts to move in opposite direction to belt speed in a belt reference frame. So belt induces kinetic friction until that object comes to rest in belt reference frame. So the same $<0$ situation here

Agnius Vasiliauskas
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work done by kinetic friction on a system is always negative with respect to ground frame. According to you,

positive work - When an object is placed on a rapidly moving belt, it moves along with the belt but with slipping (relative motion between the surfaces exist) when there is no enough friction to prevent slipping. Here the work done by the kinetic friction is positive, as the direction of frictional force and the displacement is same.

With respect to frame of belt ,work done on the object by kinetic friction is negative . But with ground frame work done on this object is +ve . But for any example /situation if you calculate the net work of force of kinetic friction on a system ,then it's always -ve . See in this situation , initially the object is at rest and kinetic friction acts on it ,so the object's initial acceleration might be less than or equal to or greater than the acceleration of belt (depending upon coeff of kinetic friction ) .Even if its equal to acceleration of belt , still kinetic friction acts unless their velocities and accelerations both become equal . (then afterwards static friction acts ) . Getting back to the point ,intially displacement of centre of mass of belt will be >> displacement of belt ,so work done by kinetic friction on system is always negative .

Raghav Madan
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Kinetic friction

  1. is a dissipative force
  2. opposes relative motion.

A "kinetic frictional force" that does no work is non-dissipative and not friction at all. Such a force has absolutely no impact on the velocity of the object.

Kinetic friction always does work for any non-zero distance.

lineage
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